The present invention relates to a device for separating two webs of material lying on top of each other, in particular those of bag bodies or individual portions thereof, while they are being transported on a conveying means, the first of the material webs being detachably fixed to the conveying means, the device comprising:
a suction means movable with respect to the second material web, in order to temporarily suck the second material web and separate it from the first material web,
two cranks spaced from each other, having a respective first rotatable joint for rotating the crank around this rotatable joint, as well as a respective second rotatable joint, to which the suction means is connected so as to be rotatable,
both cranks having the same distance between their respective first and second rotatable joints, and the distance between the first rotatable joints of both cranks being equal to the distance of the second rotatable joints of both cranks when they are connected to the suction means, so that the rotatable joints define the corners of a parallelogram.
Generally the process for manufacturing bags of any kind of material comprises a step wherein the pre-cut tubular bag bodies are folded at one end so as to form a bottom. For this purpose a slide is inserted into an open end between the walls of the flat bag body; movement of the slide, optionally cooperating with bars disposed outside the bag body, results in the desired folding of the side walls of the bag body. As the bag bodies are transported towards the slide in a state wherein they are lying flat on top of each other, there is a problem in that the seams of the open ends of the bag body or even complete border regions thereof may adhere to each other, and thus the slide cannot be inserted between the bag body walls. This effect may be caused by electrostatic charging of the material webs; by the presence of adhesive residues; in the case of plastic materials, by welding of the bag body seams in the course of cutting of the bag body by means of a hot wire; or in the case of airtight materials in general, by contiguity of areas of the material. In any case the adherence of the seams of the ends of the sack bodies makes it necessary to provide a production step wherein the end regions of the material webs are separated from each other, so as to subsequently be able to insert the slide between the material webs. Furthermore, it is desirable to carry out this separation process while continuously transporting the bag bodies so as not to decrease the production speed. Additionally, all movable parts of the device involved in the separation process are to be moving continuously, i.e. with as little acceleration and deceleration as possible, so as to avoid premature wear and the necessity of high driving energy. From the point of view of wear and energy consumption of the device, uniformly rotating parts would be ideal.
The problems mentioned may be overcome by means of various devices, for instance a device known from German Patent Application No. 1,511,021. This device has a suction bar rotating with the machine timing. The suction bar is pivotably connected to levers by means of two pins, which levers are in their turn non-rotatably connected to gearwheels by pins disposed on the latter. The gearwheels engage a further rotatable gearwheel that transmits a driving force to the former gearwheels. Thus the suction bar moves parallel to the direction of transport of hose portions. In order to linearize the speed of the suction bar, it is proposed to drive the driving gearwheel in its turn by an elliptical wheel gear. Another driving means is neither disclosed nor suggested.
A disadvantage of the device disclosed in German Patent Application No. 1,511,021 is the use of an elliptical wheel gear, which is expensive to produce, on the one hand, and which does not enable complete linearization of the suction bar movement synchronously with the movement of the bag bodies, on the other hand, but only an approximation thereof. The closer the approximation to a linearly uniform suction bar movement, i.e. the more pronounced the gearwheel ellipses, the higher the inevitable friction losses, too.
The present invention offers a solution to the disadvantages of the prior art that enables complete linearization of the suction bar movement synchronously with the movement of the bag bodies, while being highly economical.
Thus according to the invention a device for separating webs of material is provided with a drive means having an engagement element orbiting along a predetermined path and connected to an engagement element formed at one of the cranks, the orbits of the engagement element of the drive means and of the engagement element of the crank not coinciding and not being parallel, and the engagement elements engaging each other displaceably with respect to each other.
By means of the rotatable connection of the suction means with the two cranks, the rotatable joints involved forming a parallelogram, the suction means carries out a rotational movement in the course of which it approaches or contacts the second material web and sucks it, subsequently moving back again, the sucked second material web being pulled along, thus rising from the first material web. The rotational movement of the suction means is advantageous in that it has a velocity component parallel to the conveying direction of the material webs so as to guide along the suction means with the movement of the material webs in the course of the separation process, as well as a velocity component at right angles to the conveying device in order to pull the second material web away from the first one. However, in the case of uniform rotational movement, the course of both velocity components would be sinusoidal, so that the parallel velocity component of the suction means would be the same as the conveying speed of the material webs only at a certain crank angle. As a consequence, the second material web would be not only lifted from the first one, but also displaced, which would have negative consequences during subsequent processing steps of the material webs. Thus it is necessary to linearize the course of the parallel velocity component of the suction device. According to the invention, this is achieved by providing an engagement element of the driving means and an engagement element of the crank that move along paths not identical with each other, it being possible to match the parallel velocity of the suction means with the conveying speed of the material webs for a large range of crank angles by choosing the orbit paths of the engagement elements and the speed of the driving means accordingly.
With embodiments of the invention, the cranks may be formed as crank disks or with crank arms.
In order to make room for the displacements the engagement elements experience with respect to each other while orbiting, according to the invention either the engagement element of the drive means or the engagement element of the crank may be formed with a cam engaging a slot or an oblong hole or a channel in the other engagement element. Alternatively, it is also possible to provide a telescopic arm, which is articulated to the driving means and the crank.
Preferably, the suction means communicates with a vacuum source. But as the suction means only has to operate during part of its revolution, it is advantageous in this case for the communication to be interruptable depending on the angular position of one of the cranks connected to the suction means.
A convenient embodiment of a device according to the invention is characterized in that a supply pipe of the suction means is vacuum-tightly connected to a through hole in a disk, the disk being rotatable together with one of the cranks, and a stationary vacuum bar connected to the vacuum source is substantially vacuum-tightly contiguous with the opposite surface of the disk, the vacuum bar having a mouth opening taking the form of a circular arc and facing towards the disk surface, which opening coincides with the circular path described by the mouth of the through hole when the disk is rotated. In this embodiment a crank disk may serve as said disk.
A preferable embodiment of the suction means comprises a bar having an internal channel for connecting to a vacuum source and a plurality of exit openings extending from the internal channel, to which suction cups are conveniently attached.
Basically it is sufficient to drive only one of the cranks, the one to which the suction means is connected, as the driving torque is transmitted to the second crank by the suction means. It may, however, be the case that the cranks have to be set in motion from a dead center, this dead center being the crank position where the sides of the parallelogram formed by the joints of the crank coincide to form a line. In this position it is very unlikely that the cranks may be set in motion. In order to avoid this, it is advantageous to connect the two cranks for synchronous movement by means of a belt or chain gear.
The driving means with the engagement element provided thereon may be formed in different ways. According to a first embodiment the engagement element is attached to a chain or a belt encircling at least two wheels, at least one of which is driven. The chain or belt defines an orbit path of the engagement element of the driving means. Preferably the chain or belt has a run parallel to the conveying direction of the material webs, and the speed of the chain or belt is the same as the conveying speed of the material webs. Thus it is possible to achieve complete linearization of the velocity component parallel to the conveying direction of the material webs as long as the engagement element moves in the parallel run. Favorably the engagement element has a channel at right angles with the chain or belt, wherein a cam or roller engages as the engagement element of a crank.
A further embodiment of the invention is characterized in that the drive means comprises a driven disk, the rotational axis of which is offset with respect to the rotational axis of the crank it engages. The eccentricity of the driven disk and the crank connected therewith by way of engagement elements achieves the desired linearization of the parallel velocity of the suction means.
The invention will now be described in more detail by way of examples.